The General Packet Radio Service (GPRS) and its superset, Enhanced Data for Global Evolution (EDGE) for GSM, have enabled data interchange among mobile wireless subscriber device users. GPRS/EDGE permit efficient use of radio and network resources when data transmission characteristics are packet based, intermittent and non-periodic, possibly frequent with small transfers of data, for example, less than 500 octets, and possibly infrequent, with large data transfers, for example, transfers of more than several hundred Kbytes.
Public Land Mobile Network (PLMN) operators originally considered GPRS/EDGE as means for generating additional revenues based on excess capacity, for example, during non-peak usage periods. User applications were originally envisioned to include Internet browsing capabilities, electronic mail, file transfers and other applications for which best efforts data transfers were appropriate. The market for GPRS/EDGE based services however has grown dramatically and now constitutes a significant amount of network traffic during peak usage periods.
The concepts of Network Controlled Cell Selection (NCCS) and Network Assisted Cell Selection (NACS) were introduced into the GSM protocol standards to increase the efficiency of the 3rd Generation enhancement to the GPRS, thus addressing the negative impact that cell reselection has on user data throughput.
In order to maintain system simplicity, there exists no real-time handover procedure specified for best effort packet data transfers over GPRS/EDGE. Cell change is achieved in Revision 5 and prior specifications of GPRS/EDGE by simply allowing the mobile user equipment (UE) to reselect cells as it does in idle mode. This procedure causes the mobile UE to abort its packet transfer on one cell and completely re-establish the ongoing transfer on another cell.
This simple cell change mechanism is fairly effective if the new cell to which the mobile UE reselects supports GPRS/EDGE data services and has sufficient capacity to support additional packet data traffic. What frequently occurs in system operation, however, is that after the mobile UE aborts operation on the old cell and attempts to reselect to a new cell, the packet data transfer fails, often because the new cell does not have sufficient capacity to support additional packet data traffic. The mobile UE must subsequently wait and try to re-establish the data transfer. Meanwhile, the mobile UE may reselect to another cell, which may accommodate its request for a packet data transfer.
The mobile station in GPRS standby and Ready states performs cell selection autonomously by monitoring the signal strength of adjacent cells specified in a Broadcast Allocation (BA) list, which is broadcast on the Packet Broadcast Control Channel (PBCCH) or on the Broadcast Control Channel (BCCH) in the absence of the PBCCH. During packet transfer mode operation, the UE monitors the serving cell carrier and all neighbor cell carriers indicated on the BA list. A received signal strength measurement indicator (RSSI) is made in every Time Division Multiple Access (TMDA) frame on the BCCH carriers, one after another.
The reselection decision process uses the received signal level average (RLA_P), which is calculated as a running average of samples collected over a period of 5 seconds and is maintained for each BCCH carrier. The samples allocated to each carrier are distributed as uniformly as possible over the evaluation period. At least 5 received signal level measurement samples are required for a valid RLA_P.
Several criteria are used for GPRS cell reselection. A path loss criterion parameter C1 represents a minimum signal level for cell reselection for GPRS. C1 is based on the corresponding RLA_P value and represents a link budget computation based on path loss and assumptions about the RSSI of the serving cell. The mobile station reselects when C1 is 0 dBM and when other requirements are met. A cell ranking criterion parameter C32 is for selecting cells among those with the same priority. For the serving cell, C32 is equal to C1. For each neighbor cell, C32 is equal to C1 modified with cell broadcast parameters. The signal level threshold criterion parameter C31 for hierarchical cell structures (HCS) is used to assess prioritized hierarchical GPRS.
The mobile station updates RLA_P and determines C1, C31 and C32 for the serving cell and the non-serving (neighbor) cells for each new sample collected or every second, whichever is greater. Cell reselection is performed if the path loss criterion parameter C1 for the serving cell falls below zero, and if a suitable non-serving cell (see 3GPP TS03.22) is determined to be better than the serving cell. Generally, the best cell is the one having the greatest value of C32. When evaluating the best cell, hysteresis parameter values are subtracted from the C32 value for the neighbor cells. The hysteresis parameter values are broadcast on the serving cell PBCCH. For cell reselection occurring within 15 seconds of a previous reselection, the hysteresis value for RSSI is equal to 5 dB. If no suitable cell is located within 10 seconds, a cell selection algorithm is performed.
During packet data interchange, stationary or semi-stationary a mobile UE in suburban and/or urban environments may reselect substantially continuously, usually among 3 to 5 cells every 15 to 45 seconds, forcing the need for re-establishment of the packet data transfer every time reselection occurs. The high frequency of cell reselection however may adversely impact user data throughput.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below.